In recent years, most electronic devices such as computers can perform desired operations by using a variety of data. When the data is held in, for example, a semiconductor memory device (also referred to as a memory) or the like, the data can be used temporarily or permanently.
A semiconductor memory device also includes an external memory device (an auxiliary memory device) such as a hard disk or a flexible disk in a broad sense. However, a semiconductor memory device almost always refers to a semiconductor memory device such as a CPU (central processing unit).
Two main types of semiconductor memory devices are a volatile memory and a nonvolatile memory. A volatile memory refers to a semiconductor memory device in which data is lost when the power is turned off. In addition, a nonvolatile memory is a semiconductor memory device in which data is continuously held even after power is turned off and in which data can be held semi-permanently after the data is written.
Although a volatile memory has a possibility of losing data, it has an advantage of short access time. In addition, although a nonvolatile memory can hold data, it has a disadvantage of high power consumption. Semiconductor memory devices each have features in this manner, and each of the semiconductor memory devices is used in accordance with the kind or usage of data.
There are various kinds of nonvolatile memories such as an unwritable read only memory (ROM), a flash memory in which writing and erasing can be performed multiple times, and an electronically erasable and programmable read only memory (EEPROM). Of these, a write-once memory in which writing can be performed only once is preferable because data is difficult to falsify and high security can be provided in this memory.
An example of a write-once memory is an anti-fuse type memory in which voltage is applied to both terminals of an element formed using amorphous silicon to form silicide in the terminals and to short-circuit. Further, a rewritable memory such as a flash memory or an EEPROM is used and a memory region where erasure is not performed is provided, whereby the rewritable memory is logically used as a write-once memory in some cases (see Patent Document 1).
[Reference]
    Patent Document 1: Japanese Published Patent Application No. H7-297293